Light pipe is used in two main ways. In an end-light application, the light pipe is optimized to carry light along its length, and transmit it at the output face of the light pipe. In a side-light application, light is extracted out the side of the light pipe and provides illumination along its length. Often, light extracted from the side of a light pipe over the full 360 degrees around the light pipe is undesirable because a reflector would be needed to redirect a significant portion of the light towards the intended area to be illuminated. Some of the redirected light impinges on the light pipe and may be either absorbed into the light pipe so as to reduce side-light output, or is scattered into unintended directions. This is the same drawback associated with fluorescent lighting and results in an inefficient fixture for delivering light onto the target surface.
One aspect of the present invention relates to distributing light-scattering material only in a radial swath, along the longitudinal axis of the light pipe, of substantially less than 360 degrees. This results in extraction of side light in a directional manner, rather than over the full 360 degrees around the light pipe. The light-scattering material could be of the reflective type, of the refractive type, or of both types.
In accordance with one embodiment of the invention, it would be desirable to distribute light-scattering material in the core of a light pipe with a density gradient for achieving uniformity in side-light emission along the length of the light pipe. As used herein, “uniformity” in side-light emission means that the lumen output as between inlet and outlet portions of a side-light emitting section of the light pipe is within plus or minus 20 percent of the average value of each other. More uniformity than this may also be desirable in some circumstances.
In accordance with another embodiment of the invention, it would be desirable to eliminate the inefficient fixture and reflector combination for use with a light pipe by extracting the light only in the desired direction, towards a target area to be illuminated.